The importance of geological controls on the natural distribution of mercury in lake and stream sediments across Canada

The Geological Survey of Canada (GSC) has surveyed a significant portion of Canada using systematic stream and lake surveys under the National Geochemical Reconnaissance (NGR) program. Total mercury (Hg) data, available for most of the sites, reveal significant natural variation. Much of the observed variation in Hg concentration can be directly related to the composition of the bedrock, regolith and glacial deposits in the surrounding watershed. Some of the highest Hg values within the sediments of Ontario lakes occur southwest of Thunder Bay in an area underlain by shales known to be naturally enriched in Hg and other trace metals.